Microcrystalline cellulose as extrusion aid/combustible filler for alumina

ABSTRACT

The use of microcrystalline cellulose as an extrusion aid/combustible filler for rehydratable alumina allows the production of low density alumina substrates.

This invention relates to the production of low density aluminasubstrates through the use of microcrystalline cellulose as acombination extrusion aid/combustible filler for rehydratable alumina,especially wherein the final calcination treatment thereof occurs atabove 1700° F.

The production of low density alumina substrates has become increasinglyimportant in the preparation of auto-exhaust catalysts due to thenecessity for increased catalytic activity at the same time as reducedweight for the final product. Generally, it has been found that aluminabodies prepared from a rehydratable form of alumina exhibit superiorphysical properties to other known aluminas.

The standard procedures for preparing alumina substrates fromrehydratable alumina generally include: (1) partially dehydratingalumina trihydrate to make a composition having a substantial portion ofrehydratable alumina, often termed rho alumina; (2) grinding thepartially dehydrated alumina to a predetermined particle size; (3)mixing the ground particles with water and forming into shaped bodies;(4) rehydrating the shaped bodies at an elevated temperature; and (5)calcining the shaped bodies to activate and harden the alumina.

The present invention is specifically direct to the forming stage of theprocess. Formed alumina bodies have been produced by conventionalshaping methods such as beading, pelletizing or extruding. Inconventional extrusion procedures, the alumina powder is blended withwater to form a mix of a suitable extrusion consistency, and thenextruded by a standard screw type extruder through a die plate havingorifices of the desired sizes and shapes. The resultant extrudates maythen be cut to the desired lengths and further treated as detailedabove.

Rehydratable alumina upon being mixed with water, to then be extruded,will set up in the extruder to form a cementitious mass that binds theextruder screw within its barrel and defies extrusion. Accordingly,there has been considerable interest in "extrusion aids" to facilitatehandling of the rehydratable alumina. Correspondingly the desire for alow bulk density alumina having macropores has developed interest in"combustible fillers" which will be destroyed during subsequentcalcination of the extrudates.

U.S. Pat. No. 3,856,708 teaches the combined use of a finely dividedfiller material and a small amount of a suitable lubricant binder in analumina composition. The filler material is specified as being woodflour and the lubricant binder as Methocel 90 HG 100. The patent then,without exemplification or actual use, indicates the potential use ofmicrocrystalline cellulose, powdered paper pulp and finely dividedplastics in place of the wood flour portion of the fillermaterial-lubricant binder mixture.

In contrast, the present invention results in the elimination of thelubricant binder material as was required by U.S. Pat. No. 3,856,708. Ithas been discovered that when microcrystaline cellulose is used in placeof the wood flour there is no necessity for the lubricant binder. Asshown by example below when wood flour is used alone in an aluminacomposition, without a lubricant binder, the mixture sets up in theextruder and is unextrudable. To the contrary, when microcrystallinecellulose is substituted for the wood flour a readily extrudable mixtureis prepared without the further incorporation of a lubricant binder.

This invention entails the use of microcrystalline cellulose as acombined extrusion aid and combustible filler for rehydratable aluminain the absence of any lubricant binder. The resultant mixture is readilyextrudable.

The microcrystalline cellulose used herein is purified, partiallydepolymerized cellulose prepared by treating alpha cellulose, obtainedas a pulp from fibrous plant material, with mineral acids. It may beprepared as disclosed in U.S. Pat. No. 2,978,446 and is available underthe tradename Avicel® of FMC Corp. Alternatively, depending upon theavailability of materials it may be used without the drying operationspecified in the patent. It is used in amounts of from 10 to 40% byweight of the dry ingredients, preferably 10 to 30%.

The rehydratable alumina is used to provide 60% to 90%, preferably 65 to80%, by weight of the dry ingredients. Optionally, up to about 15% byweight of the dry ingredients may be recycled alumina fines ground to100% less than 325 mesh.

The above ingredients are then combined with water, which has preferablybeen cooled to between 1°-10° C. to prevent premature rehydration of thealumina. The water is used in amounts to produce extrudable mixtureshaving in the range of from about 45 to 70 percent solids, preferably 50to 57 percent.

The mixing of the ingredients with the water may be performed in anymanner desired such as through the use of a muller or a continuous mixersuch as a 4-inch twin screw Baker-Perkins 100 MPM mixer. When a mulleris used residence times of from 5 to 60 minutes are generallysufficient, whereas with a continuous mixer residence times of from 30seconds to 1 minute suffice.

After the mixing operation has provided a uniform mixture, the materialis extruded by means of a standard extruder through dies of the desiredsize and shape, and the extrudates are cut to the desired length.Preferably, the dimensions of the final calcined substrate particles arein the range of 0.125 inches in diameter and have a length/diameterratio of about 1.5.

The extrudates are then rehydrated or cured by any of the standardprocedures for so doing. Autosteaming may be carried out in coveredcontainers at temperatures of 225°-275° F. for 6 to 20 hours, dependingon the required final strength. Direct steaming may be performed byplacing the extrudates in trays in an oven preheated to about 225° F.and then injecting steam into the oven for a similar 6 to 20 hours.Alternatively, rehydration or curing may be performed by slowly dryingthe extrudates in a deep bed under conditions of high relative humidity.

After the rehydration or curing, the extrudates may be washed to removesoda. The washing, if done, is performed by standard techniques withsuch as acetic acid or ammonium nitrate in either batch or continuousprocedures.

The product may then be dried and is then calcined to burn off themicrocrystalline cellulose and thereby yield a finished substrate havinga low compacted bulk density in the order of 20 to 32, preferably 26-30,lbs./cu. ft. The calcination may be carried out in any suitable mannersuch as by either direct fired or indirect fired rotary calciners. Thecalcination temperature which should be in the range of from 1700° to2000° F., preferably 1750° to 1900° F., serves both to control andeffect both the surface area and the pore size distribution of the finalproduct.

Optionally, the rehydrated or cured extrudates may first be calcined andthen washed to remove soda. In this case the material would be dried,then calcined, then washed, and then a final drying would be required.

The resultant product has the following basic properties:

    ______________________________________                                        Pore Volume (water)  0.9-1.2 cc./g.                                           Crush Strength      >7 lbs.                                                   Crush Number        >5 lbs.                                                   Bulk Density (compacted)                                                                          20-32 lbs./cu. ft.                                        Surface Area (B.E.T.)                                                                             100-130 m.sup.2 /g.                                       Shrinkage           <4%                                                       Preferably the product has the following properties:                          Pore Volume (water) 0.9-1.1 cc./g.                                            Crush Strength      >10 lbs.                                                  Crush Number        >7 lbs.                                                   Bulk Density (compacted)                                                                          26-30 lbs./cu. ft.                                        Surface Area (B.E.T.)                                                                             100-130 m.sup.2 /g.                                       Shrinkage           <4%                                                       ______________________________________                                    

The Crush Strength is determined by compressing a single extrudatepellet between two flat plates and recording the pressure at which thefirst break of the pellet occurs. The test is conveniently performed ona Chatillon LTCM machine of John Chatillon & Sons, Kew Gardens, New Yorkwhich has a bottom plate moving up to a top plate at a predeterminedspeed of from 1/4 to 6 inches/minute. Each of the Crush Strengthsreferred to herein was performed at a rate of about 3.6 inches/minute.The test is then repeated for 99 additional pellets of the same materialand the Crush Strength being reported as the average for all 100samples.

The Crush Number is used to compare the crush strength characteristicsof different shaped pellets since the Crush Strength is dependent uponthe length of the pellet being tested. It provides a basis forcomparison of beads vs. cylinders or polylobal shapes, wherein each ofthe pellets has the same diameter. In order to equate the strengths ofthe different shapes, the length of the non-spherical sample must beconsidered. Thus the Crush Number for cylinders and polylobal shapes isdefined as: ##EQU1##

As stated, the pore size distribution is dependent upon the calcinationtemperature with a higher calcination temperature yielding a higher porevolume. Representative distributions are as follows:

    ______________________________________                                        Percent Pore Volume                                                                            Calcination Temp.                                            in Range, A Diameter                                                                           1800° F.                                                                           1900° F.                                  ______________________________________                                        <100             2- 9        2-3                                              100-200          12-20       16-18                                            200-300           4-14       11-13                                            >300             57-82       66-71                                            ______________________________________                                    

The following examples are intended to be merely illustrative of thepresent invention and not in limitation thereof. The surface area andthe shrinkage of each of the products of the examples is within theranges specified above.

EXAMPLE I

Rehydratable alumina was combined with microcrystaline cellulose(Avicel® of FMC Corporation) in a ratio of 70 parts rehydratable aluminato 30 parts microcrystalline cellulose and then mixed with sufficientwater to prepare a mixture containing 52% solids. The mixture was mulledfor 30 minutes in a Lancaster muller with a 21/2 foot diameter pan, andthen extruded using a 2-inch Welding Engineers extruder to form 0.125inch diameter cylindrical extrudates which were cut during the extrusionto a length/diameter ratio of about 1.5-1.8.

The extrudates obtained were cured by autosteaming in a closed containerat 250° F. for 16-18 hours, oven dried at 250° F. for about 2 hours, andthen calcined in an American Gas Furnace rotary calciner at 1800° F. for30 minutes to obtain a low density catalyst substrate. The resultantsubstrate had the following properties:

    ______________________________________                                        Pore Volume (water) 1.1 cc./g.                                                Bulk Density (compacted)                                                                          26 lbs./cu. ft.                                           Crush Strength      15.0 lbs.                                                 Crush Number        10.9 lbs.                                                 ______________________________________                                    

EXAMPLE II

A sufficient quantity of the formulation as prepared in Example I wasmulled in seven consecutive batches. The batches were then consecutivelyfed to a continuous extruder so that a continuous extrusion run of fourhours duration was achieved. The material obtained was cured andcalcined as described in Example I. The resultant product had thefollowing properties:

    ______________________________________                                        Pore Volume (water)  1.0 cc./g.                                               Bulk Density (compacted)                                                                          28 lbs./cu. ft.                                           Crush Strength      10.6 lbs.                                                 Crush Number         8.4 lbs.                                                 Pore Size           Distribution (%)                                          <100                 7.6                                                      100-200             12.8                                                      200-300              4.7                                                      >300                74.9                                                      ______________________________________                                    

COMPARISON EXAMPLE

The procedure of Example I was repeated except substituting 30 parts ofwood flour for the microcrystalline cellulose. The resultant mixture,having 55% solids, was found to be unextrudable, and, in fact, duringthe extrusion attempt, the extrusion die was broken.

A second attempt to extrude a 55% solids mixture of 85 parts ofrehydratable alumina and 15 parts of wood floor was also unsuccessful.

EXAMPLE III

Rehydratable alumina was combined with microcrystalline cellulose andfines in the proportion of 70 parts rehydratable alumina to 25 partsmicrocrystalline cellulose to 5 parts fines and then with water whichhad been previously chilled to 1° C., to prepare a 1000 lb. mixturecontaining 50% solids. The mixture was mulled for 5 minutes in aproduction muller, and then extruded using a 4-inch Welding Engineersproduction extruder to form 0.125 inch cylindrical extrudates. Theextrudates were cut during extrusion process to give a length/diameterratio of 1.5-1.8.

The extrudates obtained were cured by autosteaming in closed containersat about 275° F. for 16 hours, washed to remove soda, and oven dried atabout 275° F.

Representative samples of cured and washed extrudates weremuffle-calcined at 1800° F. for 45 minutes, while the bulk of theextrudate sample was calcined in a direct fired kiln at 1750° F. forapproximately 30 minutes. The resultant properties were:

    ______________________________________                                        Pore Volume (water)  --                                                       Bulk Density (compacted)                                                                          27.2 lbs./cu. ft.                                         Crush Strength       9.1 lbs.                                                 Crush Number         7.6 lbs.                                                 Pore Size Distribution                                                                            (%)                                                       <100                 8.0                                                      100-200             18.7                                                      200-300             12.3                                                      >300                61.0                                                      ______________________________________                                    

EXAMPLE IV

A combination of 75 parts rehydratable alumina, 20 partsmicrocrystalline cellulose and 5 parts fines was admixed with water,previously chilled to 1° C., to prepare a mixture containing 52.5%solids. The mixture was mulled 15 minutes, then extruded, washed andcalcined according to the procedure described in Example III. Theproduct had the following properties:

    ______________________________________                                        Bulk Density (compacted)                                                                          28.8 lbs./cu. ft.                                         Crush Strength      13.6 lbs.                                                 Crush Number         8.9 lbs.                                                 ______________________________________                                    

EXAMPLE V

Rehydratable alumina was combined with microcrystalline cellulose andfines in the proportion of 65 parts rehydratable alumina to 30 parts ofmicrocrystalline cellulose to 5 parts fines and then admixed with water,previously chilled to 8° C., to prepare a mixture containing 52% solids.The mixture was mulled for 10 minutes, then extruded as in Example III.Representative samples of the fresh extrudates were autosteamed for 16hours at 250°-275° F., oven dried at 250° F., then muffle-calcined at1800° F. for 45 minutes. A sample of this material had the followingproperties:

    ______________________________________                                        Bulk Density (compacted)                                                                          25.3 lb./cu. ft.                                          Crush Strength       8.2 lbs.                                                 Crush Number         6.3 lbs.                                                 ______________________________________                                    

EXAMPLE VI

Rehydratable alumina was combined with microcrystaline cellulose in aratio of 70 parts rehydratable alumina to 30 parts microcrystallinecellulose and then admixed with water, previously chilled to 1° C., toprepare a mixture containing 50% solids. The mixture was mulled andextruded as described in Example III. Representative samples of thefresh extrudates were autosteamed at 250°-275° F. for 12-20 hours, ovendried at 250° F., then muffle-calcined at 1800° F. for 45 minutes. Theproduct had the following properties:

    ______________________________________                                        Bulk Density (compacted)                                                                          25.1 lbs./cu. ft.                                         Crush Strength       9.7 lbs.                                                 Crush Number         7.3 lbs.                                                 Pore Size Distribution                                                                            (%)                                                       <100                 2.9                                                      100-200             16.6                                                      200-300             10.4                                                      >300                70.1                                                      ______________________________________                                    

EXAMPLE VII

A mixture of 75 parts rehydratable alumina, 15 parts microcrystallinecellulose, and 10 parts fines was blended for 15 minutes in a ribbonblender. Batch size was 440 lbs. of combined powders. The blended powdermixture and water sufficient to make a 53% solids mixture were thenseparately metered to a 4-inch, twin screw continuous mixer(Baker-Perkins 100 MPM) fitted with a 0.125 inch diameter steel die.Continuous mixing and direct extrusion were then carried out during a 90minute run. Representative samples samples of the fresh extrudates,which were cut during extrusion to a length/diameter ratio of about 2.0,were combined and then autosteamed at 250° F. for 18 hours, oven driedat 250° F. and muffle-calcined at 1800° F. for 30 minutes. The resultantproduct had the following properties:

    ______________________________________                                        Bulk Density (compacted)                                                                          26.0 lbs./cu. ft.                                         Crush Strength       7.6 lbs.                                                 Crush Number         5.0 lbs.                                                 ______________________________________                                    

EXAMPLE VIII

A mixture of 75 parts rehydratable alumina, 10 parts microcrystallinecellulose and 15 parts fines were blended for 15 minutes in a ribbonblender.

The blended powder formulation and water were then continuously mixedand directly extruded as described in Example VII during a 2.5 hour run.Representative samples of the fresh extrudates were combined and thentreated according to the procedure outlined in Example VII. The producthad the following properties:

    ______________________________________                                        Bulk Density (compacted)                                                                          27.0 lbs./cu. ft.                                         Crush Strength       8.6 lbs.                                                 Crush Number         5.7 lbs.                                                 ______________________________________                                    

EXAMPLE IX

A continuous mixing and direct extrusion run lasting 4 hours was carriedout as described in Example VII by independently metering 70 partsrehydratable alumina, 25 parts microcrystalline cellulose, 5 parts finesand water to the continuous mixer extruder. No cooling water wasrequired during this procedure. Representative samples were gathered atregular intervals, cured by autosteaming at 250° F. for 18 hours andoven dried at 250° F. The cured, oven dried samples were then combinedand calcined in a rotary calciner at 1800° F. for 30 minutes. Theresultant product had a compacted bulk density of 22.5 lbs./cu. ft.

What is claimed is:
 1. An alumina substrate consisting essentially of extrudates made from rehydratable alumina wherein the extrudates have the following properties:

    ______________________________________                                         Pore Volume (water)                                                                               0.9-1.2 cc./g.                                              Crush Strength     >7 lbs.                                                     Crush Number       >5 lbs.                                                     Bulk Density (compacted)                                                                          20-32 lbs./cu. ft.                                          Surface Area (B.E.T.)                                                                             100-130 m.sup.2 /g.                                         Shrinkage          <4%                                                         ______________________________________                                    


2. The alumina substrate of claim 1 wherein the extrudates have the following properties:

    ______________________________________                                         Pore Volume (water) 0.9-1.1 cc./g.                                             Bulk Density (compacted)                                                                           26-30 lbs./cu. ft.                                         Crush Strength      >10 lbs.                                                   Crush Number        >7 lbs.                                                    Surface Area (B.E.T.)                                                                              100-130 m.sup.2 /g.                                        Shrinkage           <4%                                                        ______________________________________                                     